Mayday! I'm locked out of Budgie!

hmm.

well one way is to move aside your dconf database - the consequence is that your desktop will return to the out-of-the box configuration and you’ll need to reapply everything again.

switch to a TTY - e.g. CTRL+ALT+F2
login with your username and password
then

mv .config/dconf/user .config/dconf/user.bak
reboot --